1. Technical Field
This disclosure generally relates to a surface emitting laser element which emits laser beams in a direction perpendicular to a surface of a substrate of the surface emitting laser element, a surface emitting laser array in which the surface emitting laser elements are arrayed, an optical scanning device using the surface emitting laser element or the surface emitting laser array, and an image forming apparatus using the optical scanning device.
2. Description of the Related Art
A VCSEL (vertical cavity surface emitting laser) emits laser beams in a direction perpendicular to a surface of a substrate of the VCSEL and has low cost, low current consumption, a small size, and high efficiency, and is suitable for a two-dimensional device, when the VCSEL is compared with an edge emitting laser. Therefore, the VCSEL has been greatly researched.
As application fields of the VCSEL, there are a light source of an optical writing system of a printer (oscillation wavelength is in a 780 nm band), a recording and reproducing light source of an optical disk device (oscillation wavelength is in a 780 nm band and a 850 nm band), and a light source of an optical transmission system using an optical fiber, for example, a LAN (local area network) (oscillation wavelength is in a 1.3 μm band and a 1.5 μm band). In addition, the VCSEL has been expected to be used as a light source between boards, inside the board, between chips in an LSI (large scale integration), and inside the chips of the LSI.
In the application fields of the VCSEL, in many cases, a laser beam output from the VCSEL (hereinafter in some cases referred to as an output laser beam) is required to have a constant polarization mode and a circular cross sectional shape.
With respect to control of the polarization mode, in manufacturing of a VCSEL using a substrate (non-inclined substrate) whose principal surface is a (100) surface, a current passing-through region (current channel region) has an anisotropic shape (for example, see Patent Documents 1 through 3).
In addition, the polarization mode is controlled by using a so-called inclined substrate (see Patent Document 4 and Non-Patent Document 1).
Further, with respect to the cross sectional shape of the output laser beam, the shape of the current passing-through region is determined to be a circle or a square by adjusting a column shape (mesa shape) of a resonator structural body (see Patent Document 5).
However, when the current passing-through region has the anisotropic shape, it is difficult for the cross sectional shape of the output laser beam to be a circle. In addition, when an inclined substrate is simply used, the shape of the current passing-through region becomes asymmetrical (see FIG. 17A), and it is difficult for the cross sectional shape of the output laser beam to be a circle. In FIG. 17B, a current passing-through region whose shape is symmetrical for two axes is shown.
[Patent Document 1] Japanese Unexamined Patent Publication No. H9-172218
[Patent Document 2] Japanese Patent No. 2891133
[Patent Document 3] Japanese Unexamined Patent Publication No. 2008-28424
[Patent Document 4] Japanese Patent No. 4010095
[Patent Document 5] Japanese Patent No. 3762765
[Non-Patent Document 1] T. Ohtoshi, T. Kuroda, A. Niwa, and S. Tsuji “Dependence of optical gain on crystal orientation in surface emitting lasers with strained quantum wells”, Appl. Phys. Lett. 65(15), pp. 1886-1877, 1994
The inventors of the present invention have studied a relationship between a shape of a current passing-through region, and a polarization suppression ratio and a radiation angle of an output laser beam in detail by manufacturing a surface emitting laser element having an inclined substrate. As a result, the inventors have newly found the following. That is, in some cases, it is difficult for a cross sectional shape of an output laser beam to be a circle, by only causing the shape of the current passing-through region to be a circle or a square.
The inventors have studied the reasons of the above results in detail and have newly found that the thickness of an oxide surrounding the current passing-through region greatly influences the radiation angle of the output laser beam when an inclined substrate is used.